1. Field of the Invention
The present invention relates to an actuator using a piezoelectric thin film, and in particular, to a micro-electromechanical device using a piezoelectric actuator.
2. Description of the Related Art
Recently, an actuator fabricated by a micro-electromechanical system (MEMS) technology has attracted attention. In the actuator, a beam is bended and displaced by a drive force, such as an electrostatic force, a thermal stress, an electromagnetic force, and a piezoelectric force. In Japanese Patent Laid-Open Publication No. 2003-181800, a shaft attached on a free end of an actuator using a piezoelectric thin film is rotated.
Moreover, micro-electromechanical devices, such as a variable capacitor and a switch, which use an actuator, has been proposed. The variable capacitor or the switch fabricated by the MEMS technology includes a movable electrode provided on a beam of the actuator having a released end supported on a substrate, and a fixed electrode provided on a surface of the substrate, which faces the actuator. The actuator varies in distance between the movable electrode and the fixed electrode.
In particular, in a variable capacitor having a piezoelectric actuator which uses an inverse piezoelectric effect or an electrostrictive effect as a drive force, a movable beam may continuously and widely vary the interval between the movable electrode and the fixed electrode. Accordingly, a capacitance variation of the variable capacitor may increase. Moreover, since the air or gas between the movable electrode and the fixed electrode is used as a dielectric, the variable capacitor has advantages of an extremely large Q value and the like.
Moreover, the structure of a variable capacitor may have applicability to a switch. For example, in a capacitive type switch, a movable electrode is capacitively coupled with a fixed electrode across an extremely thin dielectric film. Alternatively, in a DC contact type switch, a movable electrode is brought into direct contact with a fixed electrode. Such a switch fabricated by the MEMS technology has also attracted attention, since the switch has both a low on-state resistance and a high insulating isolation property in an off-state.
A piezoelectric actuator has a long and thin beam including a piezoelectric film sandwiched by upper and bottom electrodes. The beam is projected in the air. Therefore, it is a serious problem that the beam is vertically warped due to a slight residual stress in a material of the piezoelectric film. Hence, it is extremely difficult to adjust a capacitance value with an applied voltage to a variable capacitor as designed, or to set a drive voltage of a switch at a steady value.
For example, at an operation end in which the movable electrode is provided in the piezoelectric actuator, a displacement D of the beam due to the inverse piezoelectric effect is approximated by the following expression:D˜E·d31·L2·t−1   (1)where E is an electric field applied to the piezoelectric film, d31 is a piezoelectric strain coefficient of the piezoelectric film, and L and t are a length and thickness of the actuator respectively.
A warpage Dw of the piezoelectric actuator, which is caused by a residual stress occurring on the deposited piezoelectric film and the like, is approximated by the following expression:Dw˜Sr·L2·t−1   (2)where Sr is a residual stress.
As understood from a comparison between expression (1) and expression (2), both of the displacement D and the warpage Dw have a similar relation regarding the length L and the thickness t of the piezoelectric actuator. Specifically, the displacement D and the warpage Dw are proportional to a square of length L, and are inversely proportional to the thickness t. For example, in order to expand a drive range of the piezoelectric actuator, it is effective to increase the length L or to decrease the thickness t. In response, the displacement D may be increased, but also the warpage Dw is increased. Hence, in order to increase the drive range of the piezoelectric actuator with suppressing the warpage, geometrical modification for the actuator may have almost no effect. There will be no other way but to reduce an absolute value of the residual stress Sr compared to an absolute value of the piezoelectric strain (E·d31) due to the inverse piezoelectric effect.
Regarding lead zirconate titanate (PZT) that is a piezoelectric film having a large inverse piezoelectric effect, it is necessary to anneal a PZT film at about 600° C. after the PZT film is deposited at room temperature in order to obtain a good film quality. A contraction in volume may occur due to such annealing. Accordingly, a residual stress of the PZT piezoelectric film is inevitably increased. Meanwhile, for a piezoelectric film, such as aluminum nitride (AlN), zinc oxide (ZnO), and the like, which can be deposited at around the room temperature with a good film quality, it is possible to relatively precisely control a residual stress by adjusting deposition conditions. However, an inverse piezoelectric effect in the piezoelectric film, such as AlN, ZnO, and the like, is smaller than the PZT film by a factor of ten or more.
In the case of using a piezoelectric material having such a large inverse piezoelectric effect in order to increase the piezoelectric strain of the piezoelectric film of the piezoelectric actuator, it is difficult to control a residual stress in a piezoelectric film, and to suppress a warpage of the actuator. Moreover, a piezoelectric material, in which the residual stress can be controlled relatively easily, has a small inverse piezoelectric effect. In such piezoelectric material, a drive range of the actuator cannot be sufficiently increased in comparison with the warpage of the actuator. Due to the problems as described above, technological application of the piezoelectric actuator is precluded. The piezoelectric actuator is largely warped by the slight residual stress due to a serious problem relating to such a structure of the piezoelectric actuator, that is, the thin and long beam structure. Therefore, it is difficult to fabricate a variable capacitor with a capacitance held constant, or a switch with an operation voltage held constant.